Climate change is one of the greatest ecological, economic, and social challenges facing us today. The scientific evidence that human activities are contributing to climate change is compelling, and yet argument in Australia continues over whether we should act.

Understanding what science tells us about climate change and its effects is central to deciding how to deal with climate change.

Ross Garnaut’s recent updates of his 2008 climate report have emphasised that the science is even clearer now than it was when he initially reported. Confusion over whether climate change exists comes from partisan argument in the media, not from the science itself.

Yes, we do need CO2 - in moderation

We need greenhouse gases to survive. Those gases - including carbon dioxide (CO2), methane, and water vapour - retain heat and keep the planet warm enough to sustain life, much as a greenhouse does in cold climates. That’s why why we call them greenhouse gases and refer to their role as the greenhouse effect.

The sun warms the Earth’s surface and the Earth radiates energy back through the atmosphere into space. Greenhouse gases absorb some of the radiation released by the Earth’s surface and then radiate heat in all directions, including back towards the ground – adding to the heat the ground receives from the Sun.

The surface of the planet would be more than 30 degrees cooler than it is now without this natural greenhouse effect and life as we know it would not be possible.

The Earth also stores these greenhouse gases, in various forms, in the oceans, in frozen tundra, in vegetation and soils, in limestone, and in hydrocarbons such as fossil fuels. The amount and form of the gases stored in different parts of the Earth generally tends to be, on average, in balance though exchanges between different stores occurs constantly. Persistent, even slight, changes in the environment, however, especially in temperature and moisture, can lead to changes in the amounts of these gases stored in different places, such as the ocean or atmosphere.

Higher temperatures can lead to more CO2 being emitted to the atmosphere, enhancing the greenhouse effect and making the earth warmer. This warming in turn leads to more CO2 being released, resulting in more warming, and so on.

Cooling reverses this feedback cycle. These processes can lead to natural long-term fluctuations in climate in ways that are still the subject of intense research.

The effect of many human activities is to actively take greenhouse gases from one store (such as fossil fuels and forests) and move them to others, especially the atmosphere and ocean. These human “interventions” are relatively new to the Earth and have become particularly influential over the last few hundred years. We are now a significant player in the “natural” balance among the various stores of greenhouse gases that affect Earth’s climate.

Weather always changes, and so does climate

It is worth noting while thinking about climate that weather and climate refer to different things. Weather is the brief, rapidly changing condition of the atmosphere at a given place and time, mainly influenced by the movement of air masses. Climate, on the other hand, is average weather conditions over longer periods of years to decades.

Climate variability is the year-to-year variation in average conditions – for example, climate variability means this summer might be hotter than the last. Climate change refers to long-term trends in climate over many years or decades. A single warmer or cooler year is not enough evidence to conclude whether climate is changing, but systematic changes in average conditions over many years do provide evidence of climate change.

Earth’s climate has always changed, alternating between long periods of warm (interglacial) and cool (glacial) conditions, cycling over tens to hundreds of thousands of years. These changes are driven by both external influences and dynamics internal to the Earth system.

Key external influences include fluctuations in the amount of energy emitted by the Sun and changes in the Earth’s orbit and axial tilt that affect the intensity and distribution of the Sun’s energy across the Earth.

Internal influences on climate include changes in the surface reflectivity due to the presence or absence of ice, changes in atmospheric composition of greenhouse gases, variations in ocean currents, drifting continents, and the cooling effect of volcanic dust folowing significant eruptions.

The Earth in the distant past has been both warmer and cooler than today. The Cretaceous Period (120 to 65 million years ago) was 5 to 7 degrees warmer than today and CO2 concentrations were much higher than those we know. The earth cooled substantially from the Tertiary Period to the Quaternary Period (2.5 million years ago). The past million years has generally seen a series of changes from major ice ages (glacial periods) to interglacial periods about every 100 000 years, and other variations with shorter periods.

Driven by humans, happening too fast

The current change in climate is different. Firstly, this climate change is happening much faster than in the last few million years. There is now strong evidence that recent rapid climate changes are driven largely by a range of human activities, particularly those that influence the usual exchanges of greenhouse gases among the various stores on Earth (atmosphere, ocean, limestone, etc). Most of these human contributions to climate change have occurred over the last 200–300 years, following the agricultural and industrial revolutions and rapid increases in global population.

Greenhouse gases are released into the atmosphere by “new” human activities such as burning fossil fuels, clearing forests, cement manufacture, and by many other industrial and agricultural activities. Our transfer of additional amounts of these gases to the atmosphere increases the amount of radiation trapped near the Earth’s surface and drives accelerated warming. This forced transfer has no precedent in pre-human history and the associated atmospheric warming is called the enhanced greenhouse effect.

The increases in global temperatures are changing fundamental climate processes. Some of those changes may be beneficial, but most will cause more harm than good.

Climate change due to human activities is superimposed on natural climate variability. Sometimes it is masked by, and sometimes it exacerbates, these natural processes.

Historical records of temperature show that although temperatures vary naturally between ice ages and warm periods there is no record of temperatures within human history ever having increased as rapidly as they have over the past 100 years.

Many other aspects of the Earth’s climate have also changed over the past century or more. Some regions have become wetter, while others have suffered increased periods of drought. Frosts have decreased and heatwaves have increased in many parts of the world. Mountain glaciers have shrunk and the sea level has risen.

Humans have responded and adapted to small variations in climate for thousands of years but now the changes are accelerating and humanity, more than ever before, is bound to lifestyles that are dependent on immensely complex infrastructure and agriculture for continued survival. Never before, for example, have there been around 150 million people living permanently within 1 m of existing sea level. Not everyone can easily relocate to avoid rising sea level and storm surges, as many cultures might have done in the past.

We can plan for future change

Careful measurements tell us how our climate has been changing over the last 100 years. We need a way to project what our future climate will be like in future, however, to work out what we should do now to accommodate those changes yet to come. The most powerful available mechanism to make those future projections is computer modelling of the Earth’s climate system.

Years of scientific monitoring and analysis have shown the links between human activities and terrestrial, marine, and atmospheric climate processes. This analysis is central to crafting the computer models that can give us reasonable projections of future climate. These projections allow us to figure out many of the likely effects of climate change on the Earth’s systems and humanity in general. This knowledge, though sometimes imprecise, allow us to plan how to adapt to what is now inevitable change and act to prevent even greater changes.

Humanity has learned to live with climate variability and we will keep doing so. Modern climate change, however, is a whole new challenge for this generation and for those to come. Our lives and the environment in which we live are affected in myriad ways by climate: changes in that climate are about to make all our lives very different.

This article is adapted from the introduction to CSIRO’s publication, “Climate change: science and solutions for Australia”, available here.